1. Field of the Invention
The invention relates to a device for splitting a two-phase inlet stream consisting of a light phase fluid and a heavy phase fluid, for instance vapor and liquid, into two or more two-phase outlet streams. The device will ensure that the desired vapor/liquid ratio is obtained for each of the outlet streams. The total flow rates of each outlet stream do not necessarily need to be identical. The invention is suited for but not limited to the application of splitting a two-phase process stream flowing in a pipe or channel to parallel heat exchangers, furnace tubes, air coolers, chemical reactors or piping systems.
2. Related Art
Splitting a two-phase stream is required in many process units, and historically different types of solutions have been applied, ranging from use of simple symmetrical piping splits or tee's to more sophisticated two-phase stream splitters.
The devices for splitting a two-phase process stream can be divided into 6 general types:
Type 1: Symmetrical Piping Splits Using Standard Piping Tee's.
The traditional way of splitting a two-phase stream is to make symmetrical piping splits using standard piping tee's, and to rely on the phases to distribute evenly to each branch pipe. An example of a symmetrical piping split for splitting a two-phase stream into four outlet streams is shown in the isometric drawing in FIG. 1. The two-phase inlet stream flows in the inlet pipe 1. The inlet pipe 1 routes the two-phase stream to the first tee 3, where the stream is divided into two outlet streams that flow in opposite directions in a transverse pipe 7. In the illustrated example, a first or upstream 90° elbow 2 is located upstream from the first tee 3. Due to the centrifugal forces acting on the liquid, the liquid tends to flow near the large radius wall of the elbow, while the vapor tends to flow near the small radius wall. The upstream elbow 2 thus causes phase separation and a non-uniform distribution of vapor and liquid across the cross section of the pipe. To minimize the negative effect on the splitting performance in the first tee 3 caused by the upstream elbow 2, the inlet pipe 1 is preferably perpendicular to the plane defined by the tee 3 as shown. Each of the two outlet streams from the first tee 3 is further divided into two outlet streams in second and third tee's 5a and 5b, respectively. Upstream from the second tee 5a is a second elbow 4a, and upstream from the tee third 5b is a third elbow 4b. Again in order to minimize the negative effect on the splitting performance in the tee's 5a and 5b caused by the phase separation in the elbows 4a and 4b, the pipe 7 is perpendicular to the two planes defined by the tee's 5a and 5b. By use of symmetric piping splits, the inlet stream in pipe 1 has thus been divided into four product streams flowing in outlet pipes 6a, 6b, 6c and 6d. 
The symmetric piping split is probably the most widely applied method for dividing a two-phase inlet stream into two or more outlet streams. However, history has shown that this principle has failed to distribute the liquid and vapor evenly to the outlet streams, in many cases resulting in an unequal vapor-to-liquid ratio in the outlet streams. A major problem with the symmetrical piping split in standard piping tee's is that the performance of the stream split depends upon the flow regime in the upstream pipe, and that it is not always possible to stay inside the desired “dispersed flow regime” at all relevant operating conditions. The dispersed flow regime is a flow regime inside a flow channel or pipe with a uniform distribution of small liquid droplets in a continuous vapor phase or of small vapor bubbles in a continuous liquid phase (bubble flow). Also the performance of the symmetrical piping split may depend upon the presence of pipe fittings upstream from the split, as already mentioned. A major limitation of the symmetrical piping split is that the flow rate of the outlet streams needs to be close to identical to avoid significant differences in the vapor-to-liquid ratio of the outlet streams. Another limitation is that a two-phase stream can only be split symmetrically into 2, 4, 8, 16 . . . etc. outlet streams. It is thus not possible to make 3, 5, 6, 7, 9 . . . etc. outlet streams.
The performance of the symmetrical piping split in standard piping tee's has been suggested to be improved by injection of chemicals for reduction of the liquid surface tension upstream from the split. When the liquid surface tension is reduced, the dispersed flow regimes will be achieved at lower flow velocities. Therefore acceptable performance of the symmetrical piping split may be achieved over a wider range of vapor and liquid flow rates. An example is given in U.S. Pat. No. 5,190,105, where a surfactant is injected upstream from the split of a two-phase stream of saturated steam and water to a plurality of injection wells to ensure identical quality (vapor fraction) to each injection well for enhanced oil recovery from an oil reservoir.
Type 2: Use of Special Inserts such as Vanes, Baffles or Static Mixers in Piping Tee's.
Several attempts have been made to try to improve the split performance of a standard piping tee by use of pipe inserts such as vanes, baffles and static mixers.
A first example is given in U.S. Pat. No. 4,396,063, where a static mixer is located just upstream of a tee consisting of a Y-branched conduit. To achieve good splitting performance, where the vapor-to-liquid ratio of each outlet stream is identical, dispersed flow is preferred. In the dispersed flow regime the two-phase mixture will more or less act as a single-phase fluid. The small liquid droplets tend to follow the vapor flow at approximately the same velocity, or vice versa. Therefore, in the dispersed flow regime, a good splitting performance is often achieved in a piping tee. The use of a static mixer upstream from the tee provides surfaces with a certain projected area perpendicular to the flow direction in the inlet pipe. Liquid will impinge on these surfaces and will thus be separated from the vapor phase. Therefore, use of static mixers disturbs the desired dispersed flow regime, if present, and results in separation of liquid and vapor, which is unwanted. The use of static mixers introduces additional pressure drops in the process system, which may result in additional operating cost due to increased power consumption in pumps and/or compressors. Also static mixers are susceptible to fouling caused by contaminants such as scale and corrosion products.
A second example is given in U.S. Pat. No. 4,824,614. This flow splitter also includes a static mixer 22 located in the inlet pipe upstream from a tee 14 where the inlet stream 30 is divided into two outlet streams 74 and 76. Between the static mixer 22 and the tee 14, a horizontal stratifier 24 is located. The stratifier collects fluids from six different elevations. The fluids collected at the lowest and first elevation are sent to one outlet stream 76, the fluids collected at the second elevation are sent to the other outlet stream 74, the fluids collected at the third elevation are sent to the outlet stream 76, etc. As with the mixer of the first example discussed above, the mixer of the present example will tend to separate the liquid from the vapor, which is unwanted. The static mixer may also increase the operating costs and be susceptible to fouling. The stratifier collecting the fluids will only work if the vapor and liquid are distributed uniformly across the pipe cross section, which will not be the case in real applications. The Mixer/Stratifier assembly was tested in a steam/water field application described in U.S. Pat. No. 5,810,032. The result of the test was that a better split of the steam and water was obtained in a standard impacting tee than with the Mixer/Stratifier assembly.
A third example is given in U.S. Pat. No. 5,810,032. Various types of inserts for a standard pipe tee have been tested both in the laboratory with air and water, and in the field for splitting a steam/water mixture to parallel injection wells for enhanced oil recovery in an oil reservoir. Three general types of pipe inserts were investigated: A static mixer upstream from a standard tee, a vertical flow baffle upstream from a standard tee, and use of flow restrictions or nozzles in the two outlet branches of a standard tee. Combinations of these three types of inserts were also investigated. The conclusion was that the static mixer and the vertical baffle only result in marginal improvement of the split performance. The use of flow restrictions or nozzles in the two outlet branches is claimed to result in somewhat better split performance for the flow regimes tested. However it is not clear what the driving force for uniform liquid distribution to the nozzles and outlets branches of the tee is in the case of non-uniform distribution of the liquid and vapor in the cross section of the inlet pipe. None of the laboratory flow tests are carried out in Dispersed or Bubble Flow regimes (liquid droplets in a continuous vapor phase or gas bubbles in a continuous liquid phase); the evaluated flow regimes in the laboratory tests are Stratified Flow, Wavy Stratified Flow, Slug Flow, and Annular Flow, as predicted by use of the two-phase flow map by Mr. Ovid Baker (“How to size process piping for two-phase flow,” Hydrocarbon Processing, October, 1969, pp. 105-116). That is probably the reason why it was found that the split performance of a standard tee with or without inserts is better at low flow velocities and low liquid fractions. The preferred high velocity flow regimes, Dispersed and Bubble Flow, were never tested. If tests had been performed in the Dispersed and Bubble Flow regimes, the conclusion would, most likely, have been different.
Instead of using special inserts in standard piping tees, others have suggested using significantly modified tees. Examples of modified piping tees are given in JP Patent 62059397A2, U.S. Pat. No. 4,528,919 and U.S. Pat. No. 4,512,368.
Type 3: Devices which Rely on a Certain Flow Regime to be Established Upstream from the Split.
The prediction of flow regimes in industrial applications is difficult due to the lack of accuracy of the flow regime maps. Most flow regime maps are mainly based on two-phase flow regime data for air and water in small diameter piping (<2 in. or 5 cm). Therefore, for instance, in a hydrocarbon/hydrogen system at elevated pressures and temperatures, as in a hydroprocessing unit, the flow regime maps may be inaccurate.
In addition to the uncertainty in the flow regime maps comes the uncertainty in the thermodynamic models for prediction of liquid and vapor amounts and properties. This uncertainty may be significant, for instance, for complex hydrocarbon systems where the hydrocarbons are characterized by use of pseudo components and where an equation of state is used to predict the degree of vaporization and the fluid properties.
Also piping systems in process plants are often complex systems with pipe fittings like expansions, contractions, elbows, check valves, etc. Each time a two-phase stream passes such pipe fittings, the general flow regime is disturbed, and it may require long, straight pipe runs to reestablish the general flow regime. For instance, as previously mentioned, an elbow tends to separate the phases, with the dense liquid phase running near the large radius wall of the elbow, and the lighter vapor running near the small radius wall of the elbow.
For these three reasons it is normally not possible to accurately predict the actual flow regime in a pipe or flow channel. Additionally, due to variations in operating conditions such as temperature, pressure, flow rate, and chemical composition of the fluid, it is normally not possible to stay in one flow regime for all relevant operating conditions in the process unit. Nevertheless, many two-phase stream splitters are designed to work for one flow regime only.
A first example of such a two-phase stream splitter is given in U.S. Pat. No. 4,516,986. As disclosed therein, the splitter comprises an inner pipe 12 inserted in a main pipe 10. In the annular area between the inner and main pipes a baffle 13 is located. The intended flow regime in the main pipe is the Annular Flow regime where liquid is flowing in an annular ring near the pipe wall and the vapor is flowing at high velocity in the center of the pipe. Part of the liquid flowing near the pipe wall is intended to be collected in the closed end volume 14. From the closed end volume 14 the liquid is routed through an external line 15 through a control valve 23. Vapor is collected from the annular vapor volume 30 downstream from the baffle 13 and sent through a pipe branch 11, where it is combined with the liquid from the control valve. A flow meter 20 in the two-phase stream in pipe branch 11 is used to control the liquid flow. It is not described how the flowmeter can accurately measure the vapor/liquid ratio. In order to measure vapor/liquid ratio, separate flow measurements of the vapor and liquid flow would normally be required. For other flow regimes than Annular Flow (such as, for example, Slug Flow, the split performance of the device may be poor. Even if Annular Flow is the dominating flow regime in the main pipe 10, any pipe fittings such as elbows upstream from the splitter would disturb the flow. Therefore, a certain straight pipe section is needed upstream from the splitter, which may take up additional space in the process unit. Also there may be limitations in flow rate rangeability. When the total flow rate is reduced below the design value, the pressure drop across the baffle 13 is reduced rapidly, and so is the available pressure drop across the control valve 23. At some point the control valve goes fully open and is no longer able to control the liquid flow. By introducing instrumentation and control valves, the system is no longer as simple and robust as other two-phase flow splitters, and the pressure drop across the splitter is increased. A higher pressure drop normally increases the operating cost for pumping and/or compression in the process unit. The patent describes how to generate two outlet streams. If three or more outlet streams are required, then two or more splitters in series would most likely be needed. If many outlet streams are required, then the splitting system would become rather complex, and the required pressure drop would get excessive.
A second example is given in U.S. Pat. No. 4,800,921, where a horizontal header 16 is provided with outlet branches 14a, 14b, 14c, etc., and where the upstream outlet branch is at a high elevation, and the elevation of each downstream outlet branch is reduced successively. The idea should be that if Annular Flow is the flow regime in the header, then the different elevations of the outlet branches should ensure that the thickness of the annular liquid ring is approximately the same at the point of each outlet branch. Thus, the vapor/liquid ratio in each branch stream is claimed to be close to identical. As already mentioned, it is hard to predict and to stay inside a certain flow regime for all relevant operating conditions. In addition, even if Annular Flow can be maintained in the main line, the vapor/liquid ratio is expected to be a function of total flow rate in each branch line. The higher the flow rate in a branch line, the more vapor will be sucked into the pipe, and thus the higher vapor-to-liquid ratio. If the flow regime during certain operating modes is different than expected, for instance Stratified Flow, then severe maldistribution of the phases to the outlet branches is the result.
A third example is given in U.S. Pat. No. 4,574,837, where a certain phase distribution in a horizontal main pipe 10 is assumed to be known. Openings at different elevations are provided in the main pipe to allow fluids to flow first to an annular chamber 12 and then further to a branch pipe 13. The vapor/liquid ratio of the stream in the branch pipe is set by selection of appropriate flow areas of the openings at the top and the bottom of the pipe 10, respectively. The greater the flow area at the top of the pipe relative to the flow area at the bottom, the higher the vapor-to-liquid ratio that is achieved in the branch pipe. The device will only work for the Stratified Flow and Wavy Stratified Flow regimes. Also, the device will only generate a split stream with the desired vapor-to-liquid ratio when the liquid level in the main pipe is as foreseen. Consequently, the device will only work for low flow velocities and for fixed vapor/liquid ratios and properties. Most commercial applications are characterized by high flow velocities and significant variation in vapor/liquid ratio and properties.
Other examples of stream splitters which rely on a certain flow regime to be established upstream from the split are given in U.S. Pat. No. 4,574,827 and U.S. Pat. No. 5,437,299.
Type 4: Devices which Utilize Centrifugal Forces.
In U.S. Pat. No. 5,059,226, a centrifugal two-phase flow splitter is described. The centrifugal splitter has a tangential fluid inlet 28 into a swirl chamber 23. In the bottom of the swirl chamber are a central hub 38 and vanes 39 which direct the swirling vapor and liquid toward the outlet apertures 36 and into the outlet channels 37. It is not easily understandable what the driving force for distribution of the liquid phase is. The fluid inlet is not symmetrical, since there is only one inlet 28 at one side of the device. The liquid swirls along the inner wall of the swirl chamber, but, due to the asymmetric design, uniform flow and thickness of the liquid layer/film are not expected. Consequently, some of the vanes 39 are expected to collect more liquid than others, resulting in less than optimal liquid distribution to the outlet channels 37.
Type 5: Devices which Utilize an External Energy Source to Generate Dispersed Flow.
An example of such an apparatus is given in EP Patent 0003202 B1. A motor 32 and a rotating stirring device on a shaft 28 are used to disperse the liquid and vapor mixture upstream from the split where the inlet stream is split into outlet channels 4a, 4b and 4c. The device is likely to work since a Dispersed Flow regime can be generated by addition of shaft work to the shaft 28, no matter the variations in flow rates and fluid properties. The main problem with this type of apparatus is obtaining a good seal between the shaft 28 and the pipe/bend 21, which is not an easy task (not an inexpensive design) in high pressure applications like hydrocracking (up to 300 bar). Also, the initial cost, the maintenance cost of the rotating equipment, and the cost of power consumption for the motor are all high.
Type 6: Devices which First Separate Vapor and Liquid in the Inlet Stream and then Distribute each Phase to the Outlet Streams
A first example of a flow splitter for splitting a two-phase inlet stream into three outlet streams using a conventional vapor/liquid separator and conventional instrumentation is shown in FIG. 2. A two-phase inlet stream flows through a line 11 to a separator 10 where the liquid phase 13 is separated from the vapor phase 12. The vapor phase is routed via a vapor outlet line 14 to a first set of parallel control valves 15a, 15b and 15c. The position or lift of the control valves is controlled by a first set of flow controllers 16a, 16b and 16c to obtain the desired vapor flow rate through each control valve. The flow measurements are obtained by use of any conventional method, such as orifice plates or venturi tubes combined with a ΔP transmitter. The flow controllers are cascaded with a pressure controller 17. The pressure controller changes the flow set points to the flow controllers 16a, 16b, 16c in order to maintain the desired pressure in the separator 10. The liquid phase 13 is routed via a liquid outlet line 18 to a second set of parallel control valves 19a, 19b and 19c. The position or lift of these latter control valves is controlled by a second set of flow controllers 20a, 20b and 20c to obtain the desired liquid flow rate through each control valve. Flow measurements are obtained by use of any conventional method, such, as for instance, an orifice plate combined with a ΔP transmitter. The flow controllers are cascaded with a level controller 21. The level controller changes the flow set points to the flow controllers 19a, 19b and 19c in order to maintain the desired liquid level in the separator 10. Finally, the vapor streams from the valves 15a, 15b and 15c are combined with the liquid streams from the valves 19a, 19b and 19c to generate the three two-phase outlet streams 22, 23 and 24.
The instrumentation for the two-phase stream splitter shown in FIG. 2 is rather complex, and as the complexity and number of components such as transmitters, control valves and controllers are increased, the risk of failure and upsets is also increased. Some downstream systems may be damaged if the vapor-to-liquid ratio is too high or too low during such failure or upset in the control system. Examples are the risk of tube rupture or coke buildup in a furnace tube due to overheating of the tube in case the vapor-to-liquid ratio of the stream flowing inside the tube is suddenly increased. Another example is the risk of rapid coke build-up in parallel catalytic hydroprocessing reactors if the reactor is operated with too low a vapor-to-liquid ratio, resulting in hydrogen deficiency even in a short period of time. Also, the complexity of the control system and the large size of the separator vessel 10 result in a high cost of the splitter.
A second example is given in U.S. Pat. No. 4,293,025. This two-phase flow splitter includes a separator vessel 10 which has a two-phase inlet nozzle 11. An impingement plate 14 is located below the inlet nozzle to break down the high velocity of the inlet stream. Two or more chimneys 12 are provided in the separator. The upper ends of the chimneys are open to allow vapor to enter the chimney. Apertures 13 are provided in the chimneys for liquid entrance to the chimney. Caps 16 are located above the chimney openings to prevent direct liquid entrance at the chimney top. The flow of liquid to each chimney is determined by the liquid head above the apertures 13 and the flow area of the apertures. For a given liquid level in the vessel, the flow of liquid to each chimney will almost be constant. Therefore, such a two-phase stream splitter where the liquid head is the driving force for liquid distribution to the parallel outlet streams will ensure constant liquid flow to each outlet stream rather than constant vapor-to-liquid ratio. Another problem with stream splitters where the liquid head is the driving force for distribution is the limited liquid flow rangeability. The area of the apertures 13 must be sized to obtain an intermediate liquid level at the design liquid flow rate. If the liquid flow is, for example, 50% higher during some operating modes, then the liquid level will be about 2.25 times higher than the design liquid level, and liquid may thus overflow the chimneys and result in maldistribution of the liquid to the outlet streams. If the liquid flow is, for example, 50% lower than the design liquid flow, then the liquid level will only be about 25% of the foreseen liquid level. At low liquid levels, the liquid distribution performance may be poor due to a large sensitivity towards waves, non-level installation, and other fabrication tolerances. The liquid flow rangeability of the splitter can be broadened by providing apertures at more elevations. However, if apertures at more elevations are provided, then the liquid distribution performance at the design point is reduced relative to the splitter with apertures in one elevation only.
Other examples of splitters where the liquid level is the driving force for even liquid distribution to each outlet stream are given in U.S. Pat. No. 4,662,391; JP Patent 03113251 A2; and JP Patent 02197768 A2.
A third example of stream splitters with separation of the liquid and vapor phases is given in U.S. Pat. No. 5,250,104. The two-phase mixture flowing in a pipe 14 is separated in a separator 12. The vapor phase is divided into two streams in a tee 20. Each of the two vapor streams is passed through an orifice 22 and 24. The liquid is collected in a sump 30 and is passed though two parallel liquid lines 32 and 34. The pressure drop for vapor flow, ΔPV, through the orifice is almost proportional to the squared volumetric vapor velocity. The pressure drop for liquid flow, ΔPL, through the liquid lines 32 and 34, consists of a static term, ΔPSL, due to the difference in elevation of the liquid level in the sump 30 and the liquid tube ends 40 and 42, and a frictional term, ΔPFL. ΔPFL is almost proportional to the squared volumetric liquid flow rate. Since the vapor and liquid paths through the splitter are parallel paths, the pressure drops need to be identical:ΔPV=ΔPSL+ΔPFL  (1)
The flow areas of the vapor orifices and the liquid tubes are sized for a certain vapor flow rate QV and a certain liquid flow rate QL. Now if, for instance, the actual vapor flow is 50% higher during some operating modes, then ΔPV is 125% higher than foreseen. Since the liquid flow is unchanged, ΔPFL is also unchanged. In order to fulfill equation (1), ΔPSL therefore has to be increased by 1.25×ΔPV. The result is that the liquid level in the sump 30 needs to be reduced significantly, and at some point there will be no liquid level in the sump, and both vapor and liquid will enter the liquid lines 32 and 34. In such a case, poor distribution of liquid to the parallel lines 32 and 34 will be the result. On the other hand, if the vapor flow is, for example, 50% lower than the design vapor flow during some operating modes, then ΔPV is 75% lower than foreseen. In that case, the liquid level in the sump 30 will rise significantly and overflow the sump, causing liquid flow to the orifices 22 and 24 and maldistribution. The splitter will only work properly at the vapor flow rate and liquid flow rate for which it was designed. The liquid and vapor flow rangeability of the splitter is insufficient for most industrial applications, which are normally characterized by significant variation in both liquid and vapor flow rate and in liquid and vapor properties like density, viscosity, and surface tension.